A useful VFS change ...

[Jeremy Allison]

On Wed, Feb 26, 2014 at 10:28:37AM -0800, Richard Sharpe wrote:
> Hi folks,
> 
> In discussions with Dustin an idea was proposed for a way to make VFS
> writing easier for those who are accessing user-space file systems (of
> which we are seeing more and more these days.)
> 
> One of the problems is that a user-space file system cannot, in
> general, fall through to vfs_defaults, because, except in limited
> cases, these call into the kernel via syscalls with FDs that make no
> sense to the kernel.
> 
> This requires that the VFS write write code for every VFS function
> with many of them simply returning ENOTSUP.
> 
> By adding a set of flags to the structure we could simplify things.
> Perhaps we should go back to the old idea of OPAQUE vs TRANSPARENT VFS
> modules. The default is TRANSPARENT, but if the writer sets the OPAQUE
> bit, the infrastructure will return ENOTSUP if a call is made to a VFS
> function that is not implemented.

Rather than complicating the structure, shouldn't we
just create a new 'defaults' module that always returns
ENOTSUP for every call, that OEMs can then stack their
user-space filesystem modules on top of ?

Jeremy.